The principal source of helium is its separation from natural gas streams prior to the natural gas streams being used as a fuel or as a chemical feedstock. Natural gas streams can contain up to about 10 percent helium. It is economically feasible to recover helium from a natural gas stream down to a content of as low as about 0.1 percent.
The economic feasibility depends on both the capital cost for a system and the ongoing operational cost. It is an objective of the present invention to set out a system where the economic efficiency of the system is maximized through the use of a combined membrane and pressure swing adsorption system.
There is disclosed in U.S. Pat. No. 5,542,966 an efficient pressure swing adsorption process for the recovery of helium from natural gas streams. This pressure swing adsorption system will take a typical natural gas which contains from about 1 to 6 percent helium and produce a helium stream which contains more than 99.9 percent helium. This is a very high purity helium. This is done by solely using a pressure swing adsorption system.
One problem in the use of such pressure swing adsorption systems in the recovery of helium from natural gas streams which contain low levels of helium is the volumes of gas that must pass through the adsorbent beds of the pressure swing adsorption system to recover a given volume of helium. In order to accommodate large volumes of helium, the adsorbent containing columns and other equipment must be sized for this gas flow. However, as the volume of gas flow increases, the diameter and/or length of the adsorption columns is increased and the amount of adsorbent needed is likewise increased. It is this capital equipment cost plus the operating cost that will determine if a natural gas stream with a particular helium content can be used as a source of helium.
The efficiency of recovering helium from lower helium content natural gas streams can be increased if there is a membrane partial separation of the helium from the natural gas with a permeate natural gas with an increased helium content being fed to a pressure swing adsorption system. The processes of this application are directed to increasing the helium content of a natural gas stream from about two to ten times the initial concentration up to about 8 to 20 percent helium by volume in a membrane separation stage. When the helium content of the natural gas is doubled, the amount of gas to be flowed through the pressure swing adsorption system to recover the same volume of helium is halved. If tripled, the volume is reduced to one third. And when the helium content is quadrupled the amount of natural gas to be flowed through the pressure swing adsorption system to recover the same volume of helium is reduced to a quarter. This has a significant effect on the capital cost of the pressure swing adsorption system and its operating cost. There is a greater efficiency even though there is a capital cost and an operating cost for the membrane separation system. There is a net cost savings when the helium content in the natural gas stream is increased a given amount.
Using the present pressure swing adsorption system, there is no need to increase the helium content to above about 20 percent by volume. This pressure swing adsorption system can very effectively increase the helium content in this lower range to more than 99.99 percent by volume. This is not feasible with the prior art helium recovery systems.
The efficiency of the pressure swing adsorption system also can be improved. By the use of some of the high pressure residue gas from the membrane unit as the gas to aid in removing primarily nonadsorbed gas from the adsorbent bed that has completed an adsorption phase, the need for an additional compressor in the process is obviated. This results in a savings in the capital cost of the pressure swing adsorption unit.
It is known to separate helium from natural gas by means of pressure swing adsorption. Such a process is disclosed in U.S. Pat. No. 5,089,048. This patent discloses a pressure swing adsorption system for helium enrichment. The process in this patent can be used with helium streams which contain less than 10 percent helium. The process consists of a three step pressure build-up phase, an adsorption phase, a three step pressure relief phase, and an evacuation phase. In the pressure build-up phase, a cocurrent first depressurization gas is flowed cocurrently into an adsorbent bed which has been evacuated to increase the gas pressure in this bed. This is followed by a countercurrent flow of a second cocurrent depressurization gas from another adsorbent bed which has completed an adsorption phase. This is then followed by a countercurrent flow of product gas to bring the bed up to the operating pressure. This process will produce a purified helium stream but at a lower efficiency. One problem is that there is a loss of product helium in the gases that are discharged as waste gases. Since the amounts of helium in the waste gas are relatively high, their loss creates an inefficiency of the process. In the processes of the present invention, helium is maintained in the pressure swing adsorption system as a gas inventory and not removed as part of a waste gas or off-gas. In addition the multi-step pressurization and depressurization techniques are not used in the pressure swing adsorption system.
European Patent 092,695 and U.S. Pat. No. 3,636,679 also disclose pressure swing adsorption systems for helium purification. However, in European Patent 092,695, the feed gas should contain about 50 to 95 percent by volume helium. It is not suitable for gas streams containing less than about 50 percent helium, and is clearly not useful where the helium content of the gas stream is less than about 25 percent helium.
It likewise is known to separate helium from other gases using a combination of membrane separation and pressure swing adsorption separation. However, it is not known to take a helium source having a helium content of less than about 10 percent by volume helium and providing a product that is 99.99 percent by volume helium. In U.S. Pat. No. 4,701,187 the feed to the membrane has a helium concentration of 58.2 mole percent. This is a high feed concentration. In U.S. Pat. No. 4,717,407, there is described a process for the recovery of helium where the feed stream is first treated in a non-membrane unit and then in a membrane unit. The non-membrane unit can be an adsorption unit. This is the reverse of the present process and would require a greater capital cost for the adsorption unit since large gas volumes have to be processed in the adsorption unit. U.S. Pat. No. 5,006,132 discloses a process for upgrading the main product in a pipeline by means of a membrane process. The main product permeates through the membrane with contaminants rejected. The rejected gas is put back into the pipeline and the product gas is used. Helium can be one of the rejected gases. U.S. Pat. No. 5,224,350 discloses a process for recovering helium from a hydrocarbon/nitrogen stream by first removing the hydrocarbons in a liquid extraction followed by a nitrogen/helium separation in a membrane unit. The helium from the membrane unit that is in a concentration of more than 50 mole percent can be flowed to a pressure swing adsorption unit to increase the helium content to more than 99 mole percent. None of these processes is a highly efficient process. In the present system and processes, there is no need for a membrane unit to increase the helium content of a feed gas to 50 mole percent or more. The present membrane and pressure swing adsorption system and processes have a high efficiency at a feed to the pressure swing adsorption unit from the membrane unit of about 10 to 20 volume percent helium.